Priority Scheduling (Solved Problem 1)

Summary

In this video, the instructor explains how to calculate the average waiting time using the preemptive priority scheduling algorithm. The problem involves five processes with different arrival times, burst times, and priorities. The instructor walks through the process of creating a Gantt chart to track how processes are executed, preempted, and resumed based on priority. After determining the execution sequence, the waiting time for each process is calculated. The video concludes by calculating the average waiting time, which is 29 milliseconds, and highlights the importance of correctly forming the Gantt chart to solve such scheduling problems effectively.

Takeaways

• 😀 The lecture focuses on solving a problem related to preemptive priority scheduling in operating systems.
• 😀 The problem asks for the average waiting time of five processes using the preemptive priority scheduling algorithm.
• 😀 The question provides process IDs (P1 to P5), their arrival times, burst times, and priorities, where priority 0 is the highest.
• 😀 The script explains that there are no I/O burst times, so only the CPU burst times are considered.
• 😀 Preemptive priority scheduling allows higher priority processes to preempt lower priority ones if they arrive while another process is running.
• 😀 The first step in solving the problem is to create a Gantt chart to visualize the execution order of processes.
• 😀 When forming the Gantt chart, the priority of each process is checked at each arrival, and preemptions occur when necessary.
• 😀 After forming the Gantt chart, the waiting times for each process are calculated based on their start times and arrival times.
• 😀 The average waiting time is calculated by summing up individual waiting times and dividing by the number of processes.
• 😀 The final result of the calculation shows that the average waiting time for the given set of processes is 29 milliseconds, matching Option A in the choices.

Q & A

What is the focus of this lecture?

The lecture focuses on explaining how to calculate the average waiting time using the preemptive priority scheduling algorithm.

What is the key to solving problems related to priority scheduling?

The key to solving priority scheduling problems is correctly forming the Gantt chart, which helps visualize the process order and preemptions.

What does 'preemptive' mean in preemptive priority scheduling?

In preemptive priority scheduling, 'preemptive' means that a running process can be interrupted by a process with higher priority.

How is priority determined in the problem's processes?

The priority is given as a numerical value where 0 is the highest priority, and higher numbers indicate lower priority. For example, priority 0 is the highest, and priority 4 is the lowest.

What does burst time refer to in this scheduling problem?

Burst time refers to the amount of time a process requires to complete its execution on the CPU, excluding input/output operations.

What is the significance of the arrival time of processes?

The arrival time indicates when a process becomes available to execute. It is important for determining when processes can start executing based on the priority scheduling algorithm.

What is the formula to calculate the waiting time for each process?

The formula for calculating waiting time is: Waiting Time = Completion Time - Arrival Time - Burst Time.

How does preemption affect the execution of processes in this scheduling method?

Preemption allows a process to be interrupted if a new process with higher priority arrives, causing the CPU to be reassigned to the higher-priority process.

What happens when two processes have the same priority in preemptive priority scheduling?

If two processes have the same priority, the process that arrived earlier is typically given preference for CPU time.

How is the average waiting time calculated in this problem?

The average waiting time is calculated by summing the individual waiting times of all processes and dividing by the number of processes. In this case, the average waiting time is 29 milliseconds.